This invention relates to a fuel injection type multiple cylinder engine unit including an oil-fuel supplying device and an intake device provided with an improved surge tank.
In a conventional technology for a two-cycle engine of an outboard motor, for example, there has been provided an engine of an outboard motor provided with a separate type oil feeding mechanism in which a fuel and a lubricating oil are supplied through different circuits. In such an outboard motor, it is difficult to return the fuel to a fuel tank in a hull body at a time when the fuel is injected. For this reason, a vapour separator is arranged in a fuel supplying system from the fuel tank and a certain amount of the fuel corresponding to an amount consumed is fed to the vapour separator by the operation of a mechanical fuel pump through a low pressure filter. The vapour separator is operatively connected at the upper portion thereof to a delivery pipe through a return pipe to return the return fuel.
On the other hand, oil is pumped up from an oil tank and fed into the vapour separator in which the fuel and oil are mixed to form a fuel mixture, which is then supplied to the delivery pipe by an electromagnetic type fuel pump through a high pressure filter and injected into the respective cylinders from the delivery pipe through injectors.
In the separate type oil-fuel feeding system described above, the drain amount of the oil pump is controlled by the rotation speed of the engine and the degree of opening of a throttle valve and it has been aimed to reduce the amount of the oil to be consumed by changing the mixing ratio to, for example, 50/1 during high load and high rotation speed operation period and 200/1 during low load and low rotation speed operation period.
However, in the conventional separate type oil-fuel feeding system of the character described above, it is required for the vapour separator to have a relatively large volume and the injectors are disposed apart from the vapour separator. Accordingly, considerable time is required to a time at which the mixing ratio of the oil-fuel mixture injected from the injectors reaches a value suitable for the actual operation with respect to the change of the operating condition of the oil-fuel supplying system. In order to obviate this defect, in the conventional technology, an excessive amount of the oil is supplied to satisfy the lubrication, thus being uneconomical in consumption of the oil. In addition, when the injector is switched "off" during the high speed operation period, the supplying of the oil stops simultaneously with the stopping of the fuel injection. This may result in the seizure of the engine.
In another aspect, it is required for the engine mounted to the outboard motor to have a compact structure, and in order to achieve this requirement, the length of the intake passage disposed in operative association with the fuel injectors is unnecessarily shortened or a surge tank has insufficient inner volume in the conventional outboard motor. Particularly, in a case where the surge tank has an insufficient inner volume, pulsations of the mixture may be caused in the intake passage. Since, in the intake passage, the mixture is generated in the respective cylinders by the fuel injectors, there may cause a case where the air-fuel ratios in the mixtures generated in the respective cylinders may be made different when the pulsations are caused.